1. Field of the Invention
The present invention generally relates to a disk drive, and more particularly to a disk drive with dual synchronization fields in which a second synchronization field contains only a byte synchronization portion, is error tolerant and is preceded by random data.
2. Description of the Related Art
Oftentimes, problems may occur which destroy data in a data stream, or prevents it from being recovered.
For instance, one such problem is the so called “thermal-asperity problem”. If, for instance, a thermal-asperity hits the data portion of a sector, then normally the lost data is recovered using known techniques like error-correction codes (ECC). If the thermal-asperity, on the other hand, hits the synchronization field preceding the data, then the data may be lost since, if synchronization is lost, the proper timing or byte synchronization may not be present. In order to avoid such problems, some conventional disk drive systems utilize dual synchronization (hereafter “sync”) fields for each data header. Each sync field consists of two parts: a bit-synchronization part, or variable frequency oscillator (VFO) part, and a byte synchronization part (“sync” herein after). The VFO can be thought as a preamble of 1s, while the byte sync part is a vector following it and generally possessing error tolerant properties. By utilizing two sync fields as opposed to one, the problem of catastrophic failures (such as thermal asperity) affecting it is mitigated. In effect, if the two sync fields are sufficiently separated, it is expected that at least one of them will survive a thermal-asperity type of defect. However, this approach is problematic because it takes up valuable “real estate” in the stream which otherwise could be used for the data payload. That is, this conventional method, as further described below, is disadvantageous since it limits the amount of data which could be written to the disk since such a preamble of 1s (e.g., non data) must be employed.
Hence, before reading a data sector, it is necessary to achieve correct synchronization. There are two types of synchronization that must be achieved: bit-synchronization and byte-synchronization.
To this end, as mentioned above, a synchronization field is added in front of the data sector, and this synchronization field is divided into two portions. A first portion is termed a “bit-synchronization field” (e.g., also called the preamble or the VFO field), which normally includes a sufficiently long (i.e., its length depends on the application and the type of errors normally expected to be encountered) string of 1s. FIG. 1 shows a typical example of the way double sync fields are implemented today: a first sync field, consisting of a VFO portion (denoted Sync1) followed by a byte sync portion (denoted SB1), and a second sync field, consisting of a VFO portion (denoted Sync2) followed by a byte sync portion (denoted SB2). For the sake of example, we assume that Sync1 is 24 bytes (i.e., 192 bits) long, Sync2 is 16 bytes long, and both SB1 and SB2 are 4 bytes long.
Traditionally, without the double sync field, the byte synchronization portion following the VFO is generally tolerant to a limited number of errors, and once the end of this byte synchronization portion is identified, the beginning of the data can be read.
However, as mentioned above, one of the problems with this scheme is the presence of Thermal Asperities (TA). TA occurs when, for example, the read/write head is too close to the disk being read or written to. The effect of TA is a relatively long burst of errors. For example, such errors may take the form of an “empty signal” which carries no information, such as by having a loss of amplitude of the signal (e.g., no highs or lows, thereby not allowing 1s or 0s to be detected), etc. If TA occurs in data, it is expected that the errors will be recovered by the error-correcting code (ECC) protecting the data (normally, the ECC adds redundancy at the end of the data which is used for correction and detection of errors). Typically, Reed-Solomon (RS) codes are used as the ECC. Techniques like interleaving of RS codes permit the correction of relatively long bursts of errors.
As mentioned above, the problem changes when TA affects the sync field. In that case, a TA may cause a loss of either bit-sync or of byte-sync. Therefore, if the beginning of the data cannot be identified, then it cannot be determined where the bytes are.
To solve this problem, for instance, in one conventional system employing a disk system and a Lucent® Channel for reading data, a “double-sync” scheme has been implemented. Schematically, the double-sync in such a system appears as shown in FIG. 1 (i.e., the units are in bytes).
In FIG. 1, “Sync” denotes the VFO field and “SB” denotes the byte synchronization field. As shown in FIG. 1, the conventional arrangement requires 24 bytes for Sync1, 4 bytes for SB1, 16 bytes for Sync2, and 4 bytes for SB2.
Hence, in the conventional system, the Lucent channel requires a 4-byte byte sync field, although simulations by the present inventors show that very good byte sync can be achieved with only two bytes of byte sync field. This should be considered for future products not using the Lucent® channel, or even if the Lucent® channel is continued, its specifications might be modified in order to accommodate a more efficient format. Notwithstanding, there are still problems with this arrangement.
That is, in the conventional systems, a dual sync is used including a second VFO field Sync2. As a result, bit-sync can occur even in the event of TA. Further, if TA wipes out the first sync field and part of the data, the second sync field SB2 retrieves byte-sync. Obviously, there may be other causes other than TA for wiping out the byte synchronization symbol of the first field.
However, this comes at a high price of disk “real estate” being unavailable for data (e.g., payload). The highest price comes from Sync2: it uses 16 bytes, and it would be desirable to eliminate this field, provided that TAs can be handled.